1. Field of the Invention
This invention relates to a coating device, for example, to a coating device which is suitable for coating a magnetic layer upon a magnetic recording medium to be utilized as audiotapes, videotapes, etc.
2. Description of the Related Art
Recording media used in audio apparatuses, video apparatuses and computer systems include mainly so-called coated type magnetic recording media which are produced after a magnetic powder, a binder and various additives have been dispersed in an organic solvent, and the resultant mass has been kneaded to produce a magnetic paint which has been then coated on a non-magnetic substrate and dried to form a magnetic layer thereupon. This is because of the excellent productivity and wide applicability of such recording media.
The conventional method of coating a magnetic layer consists of gravure coating incorporating a gravure roll, but is often found inadequate in coating speed and in the evenness of coat thickness. To meet such problem, what has been often adopted recently is die coating which incorporates an extrusion die whereby a magnetic paint is extruded through a slit to be coated on a substrate, because this method is advantageous in that it allows the coating speed to be raised while the amount of coated paint and the evenness of resulting coat is kept controlled and maintained.
For the production of the coated type magnetic recording medium, to improve the magnetoelectric conversion of the product, it is customary to submit, after a magnetic paint has been coated on a non-magnetic substrate, the resulting coated substrate which is still wet, to a surface smoothing treatment. If the surface of magnetic layer formed on a non-magnetic substrate does not have a sufficient surface property, particularly if the coated film has an uneven thickness owing to faulty coating, the resulting recording medium will give an inadequate signal recording capability. To avoid such inadequacy, a technique has been generally known in which a bar smoother to act as a surface smoothing means is allowed to contact mechanically with the coated surface to smoothen that surface.
Particularly, with the above described die coating incorporating an extrusion die, coating is performed while the die is kept immobilized, and thus surface roughness consisting of minute longitudinal lines in the direction along which the non-magnetic substrate runs easily develops on the surface of the coated layer. The severity of this surface roughness depends on the magnitude of shearing force the paint receives at the coating unit, and is larger with the die contact coating where die is allowed to contact with the substrate than with the die lift coating where die does not contact with the substrate. Moreover, surface roughness tends to develop more easily as the magnetic powder becomes more fine to make the resultant magnetic recording medium more suitable for a high density recording, because the fineness of powder encourages the aggregation of the resulting paint. These effects are more manifest with die coating.
For a conventional die coating to be free from such flaws as described above, it is necessary (particularly with the die lift coating) to combine the die and bar smoother appropriately so that the surface of coated layer may be flawlessly smoothened.
Still, smoothing a coated layer with a bar smoother has following three problems and remains to be further improved.
(1) If a foreign substance(s) is captured by the smoother which is in contact with a magnetic paint, it may cause a line to form on the coated magnetic layer.
(2) Each time coating is completed and the smoother is wound into a roll, it will be necessary to clean the surface of bar, which will reduce the workability of the device.
(3) Both margins of a substrate will face the edges of smoother where paint may be not present, and receive scratches from the bare edges, or will easily suffer deteriorated coating owing to the paint mass which has been dried and adhered to the bar.
To replace such conventional bar smoother, Japanese Patent Laid-Open No. 60-202542 disclosed a technique which consists of placing a smoother incorporating magnets downstream of the roller to coat a magnetic paint. The use of such a smoother with magnets seems, on outlook, to achieve complete smoothening of the surface coated with a magnetic paint, but, in reality, smoothening is inadequate with this method because the method consists simply of moving the magnets normal to the direction along which a substrate runs. Therefore, the surface smoothness of the magnetic layer is not improved by this method. In addition, with this method, a magnetic paint is coated with a roll, and thus the coating speed is slow as discussed above, and furthermore the evenness of coat surface is readily impaired.